Particulate nitrocellulose coated with sorbitan trioleate



United States Patent 3,118,797 PARTKIULATE NITRGCELLULQfiE QOATED WITHStildllTAl l TRHQLEATE Ralph E. Coliec. Alton, llh, assigncr to GlinMathieson (ihernicai Corporation, a corporation of Virginia No Drawing.Filed May 1, B62, Ser. No. 191,426 4 Claims. (Cl. 1149-19) Thisinvention relates to improved propellant powder compositions and toimprovements in the process for preparing them.

Smokeless powder compositions have been used extensively in thepreparation of propellant grains for use in rocket motors, missiles, andgas generating devices of various types. For example, U.S. Patent No.2,916,775, issued to John J. ONeill on December 15, 1959, discloses aprocess for preparing large propellant grains, wherein finely dividedparticles of nitrocellulose base propellant are mixed with a plastisolsolvent consisting of relatively non-volatile nitrocelluloseplasticizers to form a flowable paste. The paste is then formed into theshape of a rocket propellant grain and maintained in the shape untilconsolidated. In processes such as this it is necessary to handle finelydivided particles of nitrocellulose powder, which have an averagediameter of about 0.003 inch. This powder, because of its small size, isextremely dust which causes severe health and safety problems. In orderto provide a clean atmosphere in which personnel can work, it isnecessary to employ expensive ventilating equipment in the area in whichthe nitrocellulose powder is handled and processed in a dry form. inaddition, the finely divided nitrocellulose particles are verysusceptible to the accumulation of static charges which can result in anuncontrolled explosion of the powder. Therefore extreme care must beexercised to avoid the generation of a spark in this material.Furthermore, the finely divided nitrocellulose particles are not readilywetted by the plastisol solvent used in the preparation of the rocketpropellant grains. As a result, mixing of the finely dividednitrocellulose particles and plastisol solvent is extremely difiicult,since prolonged mixing is necessary to uniformly wet the solid particleswith the plastisol solvent and obtain a rocket propellant grain havinguniform ballistic properties.

It is the primary object of this invention to overcome the disadvantagesinherent in nitrocellulose powder available heretofore.

Another object of the invention is to provide nitrocellulose powderparticles which are substantially dustfree while retaining their treeflowing characteristics.

Still another object of this invention is to provide an improved astaticpropellant powder charge.

Another object of the invention is to provide finely dividednitrocellulose powder particles having improved wettability whencontacted with a casting plastisol solvent.

Still another obiect of the invention is to provide a method ofinhibiting foam formation in globular propellant powder processes whileproducing a solidified powder that is substantially dust-free.

It has now been discovered that when nitrocellulose powder in finelydivided form is coated with sorbitan trioleate to yield a substantiallycomplete uniform coating or layer of sorbitan trioleate on each particleof the nitrocellulose powder, the nitrocellulose powder maintains itsfree flowing properties, but dusting and accumulation of static chargeson the nitrocellulose powder are substantially minimized.

The proportion of sorbitan trioleate on the surface of the particles ofnitrocellulose powder is maintained between about 0.02 percent and about0.5 percent by weight of the nitrocellulose powder. When the proportionof Fatentod Jan. El,

"ice

sorbitan trioleate is less than about 0.02 percent by weight of thepowder, generally there is insufficient sorbitan trioleate present toimpart the desired improvements in the dusting and static properties ofthe nitrocellulose powder. On the other hand when the proportion ofsorbitan trioleate on the powder surfaces is greater than about 0.5percent by weight, little additional improvement is obtained in thedusting and static properties, and the excess sorbitan trioleate acts asa diluent for the nitrocellulose powder. When sorbitan trioleate isadmixed with nitrocellulose powder particles in the aboveunentionedrange, a suilicient amount of sorbitan trioleate is present to impart asubstantially complete uniform coating or layer of sorbitan trioleate oneach particle of nitrocellulose powder. The sorbitan trioleate has noappreciable solvent effect upon the nitrocellulose, an; it does notpenetrate the exterior surfaces of the grain, but instead is presentonly as an external coating having approximately a molecular thickness.

Sorbitan trioleate may be applied to the surfaces of the nitrocellulosepowder in any convenient manner that will yield a substantially completeuniform coating or layer of the sorbitan trioleate on the exteriorsurfaces of the nitrocellulose powder particles. Thus, thenitrocellulose particles can be mixed with the sorbitan trioleate in theabove defined proportions in a suitable tumbling apparatus, with orwithout water. If desired, the nitrocellulose powder particles and/ orthe sorbitan trioleate can be mixed with water prior to the coatingoperation. In one embodiment of the invention, an aqueous slurry orsuspension of nitrocellulose powder particles to be coated is admixedwith an aqueous emulsion or suspension of sorbitan trioleate in acontinuous process. This is most readily accomplished by dewatering anaqueous slurry of the nitrocellulose powder particles in a continuouscentrifuge, and then adding an aqueous emulsion of sorbitan trioleate tothe centrifuge. The emulsified sorbitan tri oleate readily replaces thewater wetting the nitrocellulose powder, and the excess emulsion isconveniently removed from the powder charge by continued centrifugation.Any water remaining on the powder grains can then be removed byconventional drying methods.

The novel coating technique of the instant invention is particularlysuitable for use in conventional ball powder processes wherein a lacquerof nitrocellulose dissolved in a suitable solvent, is dispersed in anaqueous medium to form globules of the nitrocellulose lacquer, and thesolvent is then evaporated from the globules to form solidifiedparticles of nitrocellulose.

Prior to the hardening process, protective colloids such as animal boneglue are added to tie lacquer suspension, which may cause considerablefrothing or foaming of the suspension. Such frothing adversly ailectsthe operation of the pumps, agitators, and other equipment employed inthe process. However frothing is substantially eliminated, and thedesired coating is eliected when sorbitan trioleate is added in theabove defined proportions to the hardening step in conventionalnitrocellulose powder processes. For example, when sorbitan trioleate isadded to the flash chamber in the process described in US. Yatent No.2,888,713, issued lune 2, 1959, to Ralph L. Cook and Eugene A. Andrew,or to the still in US. Patent No. 3,814,246, issued December 26, 1961,to the same inventors, frothing is substantially eliminated, and thesolidified nitrocellulose particles produced in these processes, whencombined with the sorbitan trioleate addition of the instant invention,has a substant ally complete uniform layer of sorbitan trioleate on theexterior surfaces.

The size or shape of the nitrocellulose powder particles is immaterial,but the instant novel invention is utilized more advantageously withspherical nitrocellulose powder grains having an average diameter ofbetween about 0.0002 and about 0.004 inch. Nitrocellulose particles ofthis size are used as the base material in the manufacture of giantrocl'et grains and the like. Although the present invention is concernedprimarily with the treatment of such casting powder, the qualities ofstandard small arms ammunition propellant are also materially improvedwhen coated with sorbitan trioleate in accordance with the instant invenion. Sorbitan trioleate can be used to replace or augment conventionalgraphite glaze on commercial and military nitrocellulose powders.

When nitrocellulose powder particles coated with sorbitan triolcate inaccordance with this invention are used as a base material in themanufacture of rocket grains, the nitrocellulose particles are mixedwith a plastisol solvent which has relatively poor solvent power for thenitrocellulose at ordinary room temperature, but which is very rapidacting and has a relatively great solvent power at elevatedtemperatures, for example at temperatures above about 60 C. Theplastisol solvent can be either a deterrent or an energizing modifier,or a mixture of energizing modifier and deterrent. Suitable energizingmodifiers include nitroglycerine, triethylene glycol dinitrate, butanetriol trinitrate, diglycol dinitrate, ethylene glycol dinitrate, and thelike. Suitable deterrents that may be employed include dibutylphthalate, dimethyl sebacate, dibutyl succinate, dibutyl adipate,dipropyl adipate, triacetine, ethyl diphenyl phosphate, tributylphosphate and the like. Sorbitan trioleate of the instant invention isreadily compatible with plastisol solvents of this type and its usemarkedly reduces the time required for obtaining a uniform plastisolmixture. In addition, sorbitan trioleate has no appreciable solventpower for nitrocellulose powder, and does not adversely afiect the potlife of the plastisol mixture.

In order that the invention may be further illustrated, the followingexamples are presented without any intention of being limited thereby.All parts and percentages are by weight unless otherwise specified.

Example I Nitrocellulose lacquer was prepared by adding about 1.5 partsof nitrocellulose-water mixture (1 part nitrocellulose and 0.5 partwater) to 4 parts of ethyl acetate containing about 1 percentZ-nitrodiphenylamine and about 0.25 percent of chalk. These materialswere then mixed at a temperature of about 65 C. for about /2 hour toform a substantially homogeneous lacquer. In a separate vessel asuspending medium was prepared by admixing 3.4- pcrcent sodium sulfate,1.7 percent animal glue, 0.5 percent ethyl acetate, 0.00235 percentsorbitan trioleate with water. The resulting mixture was then heated toabout 65 C. with agitation until substantially complete dissolution waseffected.

The nitrocellulose lacquer and the aqueous suspending medium wereseparately fed to a grainer of a type similar to that shown in FIGURE 2of US. Patent No. 3,014,- 246. The nitrocellulose lacquer was fed to thegrainer at a rate of about /2 that of the aqueous suspending medium. Thenitrocellulose lacquer was divided by the rotating blades of the grainerinto fine globules having an average diameter of about 0.010 inch, andthe resulting granules were uniformly suspended throughout the aqueoussuspending medium. A portion of the resulting aqueous suspension ofglobules was continuously withdrawn from the grainer and admixed withadditional aqueous suspending medium (in a ratio of 5 parts to 1 part ofthe original suspension) which had been heated to a temperature of about128 C. and was under a pressure of about 4-0 p.s.i. The diluted aqueousglobule suspension was then passed through an orifice into a flashchamber maintained as slightly below atmospheric temperature. Ethylacetate and some water vapor were evaporated from the diluted aqueousglobule suspension when the pressure of the aqueous suspension wasreduced. Substantially no foaming occurred in the flash chamber due tothe presence of sorbitan triolate. These Vapors were removed from theflash chamber under a slight vacuum. Due to evaporation of the ethylacetate, the globules in the aqueous medium were solidified intogelatinized nitrocellulose particles. The resulting slurry of theseparticles, which were coated with a thin substantially continuous layerof sorbitan trioleate, was fed to a centrifuge to remove the aqueousmedium. The particles were then washed with water and heated to dryness.The dried nitrocellulose particles contained about 0.2 percent sorbitantrioleate as a coating. The coated particles had a diameter betweenabout 0.0002 and 0.004 inch, but were not dusty when stirred in theatmosphere.

Example II The superior defoaming properties of sorbitan trioleate weredemonstrated in a series of comparative tests using an aqueous solutioncontaining about 3.4 percent sodium sulfate, about 1.7 percent animalglue, and about 0.5 percent ethyl acetate.

One drop of sorbitan trioleate was added to 25 milliliters of theaqueous solution in a milliliter test tube and the test tube wasstoppered and vigorously shook eight times. The resulting foamcompletely dissipated in less than 5 seconds.

For purposes of comparison, when the procedure was repeated, using onedrop of sorbitan monooleate instead of sorbitan trioleate, more thanseconds were required before the foam completely dissipated.

For purposes of further comparison, the procedure was repeated with theexception that nothing was added to the standard solution prior toshaking. Fifty-two seconds were required for the foam to completelydissipate in this instance.

While the invention has been described in detail with respect to thecoating of nitrocellulose particles it will be recognized by thoseskilled in the art that nitrocellulose may be used alone or incombination with deterrents such as dinitrotoluene, dialkyl and diarylphthalates, and the like. In addition energizers such as nitroglycerine,aluminum, trinitrotoluene, cyclotrimethylenetrinitramine, ammoniumperchlorate, and the like can also be incorporated with thenitrocellulose or mixed with the surface coated powder. Wheneverdesirable, the nitrocellulose can be replaced in whole or in part byother gelatinizable polynitro materials such as nitro starch, polyvinylnitrate, and other similar materials.

It will be recognized by those skilled in the art that variousmodifications, some of which have been referred to above, may be made inthe invention without departing from the spirit thereof.

Having thusly described the invention, what is claimed and desired to besecured by Letters Patent is:

1. Nitrocellulose particles coated with sorbitan trioleate in aproportion equivalent to between about 0.02 and about 0.5 percent byweight of the nitrocellulose, the coat ing of sorbitan trioleate beingsubstantially complete and of substantially uniform thickness.

2. The process for preparing substantially dustless nitro celluloseparticles which comprises admixing introcellulose particles withsorbitan trioleate in a proportion equivalent to between about 0.02percent and about 0.5 percent by weight of the nitrocellulose, andcontinuing the mixing until a substantially complete layer of sorbitantrioleate is formed on the outer surfaces of the nitrocelluloseparticles.

3. The process or claim 2 wherein sorbitan trioleate is mixed with thenitrocellulose particles as an aqueous dispersion.

4. In the process for preparing solidified nitrocellulose particleswherein lacquer globules of nitrocellulose in a solvent are suspended inan aqueous non-solvent medium to yield a suspension of the globules, thesuspension is subjected to a temperature above the boiling point of thesolvent and a pressure sufficient to prevent boiling of the solvent atthe temperature employed, the resulting suspension is conveyed to areduced pressure zone Where temperature and pressure conditions are suchthat the solvent is evaporated from the globules to yield an aqueoussuspension of solidified particles of gelatinized nitrocellulose, andthe solidified particles are then separated from the aqueous suspension,the improvement which comprises adding sorbitan trioleate to thesuspension of lacquer globules in a proportion equivalent to betweenabout 0.02 and about 0.5 percent by Weight of nitrocellulose containedtherein, whereby foaming is inhibited in the reduced pressure zone andsaid solidified particles are substantially dustless.

References Cited in the file of this patent UNITED STATES PATENTS2,771,351 Hohne Nov. 20, 1956 FOREIGN PATENTS 579,302 Great Britain July30, 1946 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNoe 3 118 797 January 21 1964 Ralph Ea Coffee It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 3 line 73 for "as" read at column 4 line 3 for "triolate" readtrioleate lines 62 and 63 for "'introcellulose" read nitrocelluloseSigned and sealed this 23rd day of June 1964.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents ERNEST W. SWIDER AttestingOfficer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No,3 118 797 January 21 1964 Ralph Ea Coffee It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 3 line 73 for "as" read at column 4 line 3 for "triolate" readtrioleate lines 62 and 63, for "introcellulose" read nitrocelluloseSigned and sealed this 23rd day of June 1964,

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents ERNEST W. SWIDER AttestingOfficer

1. NITROCELLULOSE PARTICLES COATED WITH SORBITAN TRIOLEATE IN APROPORTION EQUIVALENT TO BETWEEN ABOUT 0.02 AND ABOUT 0.5 PERCENT BYWEIGHT OF THE NITROCELLULOSE, THE COATING OF SORBITAN TRIOLEATE BEINGSUBSTANTIALLY COMPLETE AND OF SUBSTANTIALLY UNIFORM THICKNESS.